Ok, since @brixen was going to write the faq I created this issue here. Move it elsewhere if you think there's a better place.
In #1256 @dbussink mentions new specs that might arise when fixing a failing spec for 1.9 mode. Maybe add a reminder for that in the faq, too.
I wrote a quick post on how to write a fix for 1.9 mode and where things move (xxx.rb -> xxx18rb | xxx19rb) https://gist.github.com/1289312
I think this post basically covers the points here http://rubini.us/2011/10/18/contributing-to-rubinius/